A barium sulfate powder is often used as a filler of a resin composition for light reflection applications. More specifically, it is used for a back sheet of solar cells (Patent document 1), a reflection film of a backlight unit using a LED as a source of light such as liquid crystal television, and a resin composition for a case body of LED. For these applications, high whiteness is required.
To obtain barium sulfate powder having a high whiteness degree which can be used for these applications, barium chloride and barium hydroxide have been used conventionally as a starting material. However, the whiteness degree of a barium sulfate powder obtained by using barium chloride as a staring material is high but a large quantity of residual chlorides as byproduct salt is generated. On the other hand, the whiteness degree of a barium sulfate powder obtained by using barium hydroxide as a starting material is high and a content of corrosive components such as sulfur components and chlorides is low but a volatile content is more than 0.5 wt % and the problem is that electronic parts of solar cells and LED are affected by it.
To solve the problem, a barium sulfate powder synthesized by using barium sulfide as a raw material has attracted attention. The barium sulfate synthesized by using barium sulfide as a material is used not only in a coating but also especially suitably in resin compositions for an ink, a film and a sheet requiring a high dispersibility because of superior dispersibility. In addition, a content of elements such as chlorine and sodium in the barium sulfate powder is a few so that problems such as insulation reduction and corrosion of electronic devices are hardly caused when it is used as a filler. Further, barium sulfide is cheaper than barium chloride and barium hydroxide so that barium sulfide has a cost merit.
However, the requirement to the reliability of the electronic parts has been increased and minor sulfur component contained in the barium sulfate powder which is synthesized by using barium sulfide as a raw material is considered as a problem. For example, an ink and a resin composition containing the barium sulfate powder which is synthesized by using barium sulfide as a raw material may deteriorate and corrode a metal part such as an electrode in electronic parts so that the function, the durability, and the reliability of the electronic parts and solar cells and LED containing the electronic parts may be impaired. Further, the whiteness degree of the barium sulfate powder becomes low when a content of sulfur components is high. Therefore, the barium sulfate powder obtained by using barium chloride and barium hydroxide as raw material has been used even though it is expensive.
In addition, there is a problem of bad smell derived from the sulfur components. The barium sulfate powder is used in various purposes such as a coating, an ink, and an additive to a resin and a rubber. In all purposes, in processes of dispersing barium sulfate powder containing the sulfur components into water, a solvent, or a resin, an operating environment may be deteriorated by the generation of bad smell even if the degree of bad smell is extremely low.
As a method for producing a barium sulfate powder using barium sulfide as a raw material, a method for obtaining a barium sulfate powder using barium sulfide, ammonium sulfate, and sodium hydroxide is proposed (Patent document 2). However, in the barium sulfate powder obtained by such method, sodium is remained and the cost is increased because of treating byproduct nitrogen compounds.
It is known that a volatile matter content is reduced by heat treating the barium sulfate powder derived from barium sulfide (For example, [0013] and [0027] of Patent document 3, claim 4, and examples 1 to 12 of Patent document 4, and [0010] of Patent document 5). However, in these prior documents, the treating temperature is not be specified, or only a treatment with a relatively low temperature, being 300 to 800° C., is disclosed even if the temperature is specified. In such a method, it is impossible to reduce sufficiently a content of sulfur components and improve the whiteness degree. In addition, within the above-mentioned range of temperature, it is considered that powders with a high whiteness degree cannot be obtained because the whiteness is deteriorated as mentioned later according to the retention time.